Remove determining time zone via systemsetup on macOS

According to https://truesecdev.wordpress.com/2015/04/09/hidden-backdoor-api-to-root-privileges-in-apple-os-x/comment-page-1/
the `systemsetup` command has required superuser privileges since
macOS 10.8.5 (which has been EOL for over 6 years at the time of writing).
We shouldn't expect to use a codepath that requires a helper tool
that requires su in any regular use; IOW, _if_ a Babel user had ever
reached this path without having been superuser, it would have failed anyway
on any currently supported version of macOS.

Closes #895 (supersedes it).

babel/localtime/_unix.py

@@ -1,10 +1,6 @@
 import os
 import re
-import sys
 import pytz
-import subprocess
-
-_systemconfig_tz = re.compile(r'^Time Zone: (.*)$', re.MULTILINE)
 
 
 def _tz_from_env(tzenv):
@@ -57,26 +53,6 @@ def _get_localzone(_root='/'):
             except pytz.UnknownTimeZoneError:
                 pass
 
-    # If we are on OS X now we are pretty sure that the rest of the
-    # code will fail and just fall through until it hits the reading
-    # of /etc/localtime and using it without name.  At this point we
-    # can invoke systemconfig which internally invokes ICU.  ICU itself
-    # does the same thing we do (readlink + compare file contents) but
-    # since it knows where the zone files are that should be a bit
-    # better than reimplementing the logic here.
-    if sys.platform == 'darwin':
-        c = subprocess.Popen(['systemsetup', '-gettimezone'],
-                             stdout=subprocess.PIPE)
-        sys_result = c.communicate()[0]
-        c.wait()
-        tz_match = _systemconfig_tz.search(sys_result)
-        if tz_match is not None:
-            zone_name = tz_match.group(1)
-            try:
-                return pytz.timezone(zone_name)
-            except pytz.UnknownTimeZoneError:
-                pass
-
     # Now look for distribution specific configuration files
     # that contain the timezone name.
     tzpath = os.path.join(_root, 'etc/timezone')